In recent years, with the increase in the energy demand on a worldwide scale, there has been a rapid increase in demand for buildings such as structures of energy-related facilities in cold climate areas. These facilities include, for example, a floating production, storage and offloading system (FPSO), in other words, a facility that produces oil and gas at sea, stores the product in a tank within the facility, and directly offloads it to a transport tanker. H-beam steels used in building these structures are required to have excellent low-temperature toughness.
Conventionally, H-beam steels have been used in a general building structures, and H-beam steels having excellent toughness and fireproof have been proposed (see, for example, Patent Documents 1 to 3). For general building structures, Charpy absorbing energy at approximately 0° C. is required. On the other hand, for H-beam steels used in the energy-related facilities in cold climate areas, Charpy absorbing energy, for example, at −40° C. is required. Further, in order to rationally guarantee the low-temperature toughness, it is necessary to specify CTOD values at −10° C. in addition to the characteristics of Charpy impact tests.
The crack tip opening displacement (CTOD) test is one for evaluating fracture toughness of a structure containing imperfections. When bending stress is applied to a test piece having a crack while predetermined temperatures are maintained, the phenomenon of “unstable fracture” occurs in which a crack rapidly propagates. With this CTOD test, the crack tip opening displacement (CTOD value) immediately before this crack rapidly propagates is measured. Favorable correlation may not always exist between the CTOD value and the Charpy absorbing energy.
In particular, if H-beam steels are manufactured by applying hot rolling to blooms obtained through continuous casting, it is difficult to secure toughness through reduction in the size of crystalline grain. This is because the maximum thickness of the bloom that continuous-casting equipment can manufacture is limited, and hence the rolling reduction is insufficient. Further, if rolling is performed at high temperatures to obtain products with high dimensional accuracy, the thick flange portion has high rolling temperatures, which leads to a decrease in the rate of cooling. This causes a concern that, at the flange portion, crystalline grains coarsen and toughness deteriorates. Although structures having fine grains can be obtained by applying accelerated cooling after rolling finishes, an enormous cost is required to install such equipment.